Project Summary DNA double stranded breaks (DSBs) are catastrophic events caused mainly by exposure to ionizing radiation that can lead to mutations, chromosomal rearrangements, genome instability, and ultimately cancer. The MRN protein complex and ATM kinase are central players involved in sensing a DSB and initiating a cellular response that leads to either repair of the lesion, apoptosis, or senescence. Despite their known importance in DSB sensing and repair, little is understood regarding how MRN assembles on DSBs, how ATP-driven conformational changes in MRN aid in DSB repair, and how MRN recruits and activates ATM kinase. This proposal seeks to utilize state-of-the-art cryogenic electron microscopy (cryo-EM) techniques, along with rigorous biochemical assays to gain high-resolution structural and function insights into these key processes. These studies will provide both a more complete understanding of this pathway, and will pave the way for future structure guided drug design efforts targeting the MRN:ATM interaction in cancer cells. !